As we noted in the previous section, Earth is but one of the many spherical bodies circling the Sun that include "planets" and "moons". By convention, a "moon" is a planet circling one of the "main" planets going around the central star. Some moons are larger than some main planets. Earth belongs to the "inner" planets, that is, the planets inside the asteroid belt, and it is part of the group of "terrestrial" planets, that is, those with a hard surface of rock and an atmosphere. Its closest neighbors are the Moon, Venus and Mars, and all three are to some extent similar to Earth. While, as mentioned, the Moon has no atmosphere to speak of, it has rocks that are more or less familiar from Earth (basaltic rock on the sea floor). Venus and Mars have atmospheres above a rocky surface. None of the neighbors have a layer of water such as Earth's ocean, and none have an atmosphere made of nitrogen and oxygen. Instead, the most abundant gas in the atmospheres of Venus and Mars is carbon dioxide. This gas is present in Earth's atmosphere as a trace gas and is intimately involved in the heat budget of the planet and in life processes (as are nitrogen and oxygen).

The single most striking feature setting Earth apart from its neighbors is the layer of water covering most of the rocky surface. Why does Earth have an ocean, while Moon, Mars, and Venus do not?

A satisfactory answer to this deceptively simple question can only come from a careful consideration of the evolution of each planet. For now, we might say, crudely simplifying, that the Moon and Mars are too small, and Venus is too hot for an ocean. Also, Mars is rather cold, which might support a layer of ice but not a layer of water, even if it were large enough and had the requisite gravity to retain water.

To illustrate the simple (perhaps overly simple) concepts behind the answer to the question why Earth has an ocean and its sibling planets do not, let us make some interplanetary thought experiments. (Of course, such experiments are entirely impossible, except in science fiction.)

First, let us reduce gravity on Earth to that of the Moon (whose mass is one eightieth of the home planet). Immediately, the air would start to get thinner, as the gas molecules of the outer atmosphere would have sufficient velocity to leave the planet. Air pressure would continue to drop, and the ocean would release its own gas content, mainly carbon dioxide, and would slowly evaporate in an attempt to replenish the atmosphere. Inexorably, the atmospheric molecules would escape into space, until the ocean would be used up. True, a drastic drop in temperature (as a result of losing the greenhouse protection of the atmosphere) would slow the evaporation of the ocean, as the water would turn to ice. But the ice would still sublimate in the sunlight, in the low pressure of the vanishing atmosphere.

Next, let us make Earth the size of Mars (nine times less mass) and move it into the orbit of Mars. Again, we would see a thinning of the atmosphere as a result of loss of gas to space, but now much more slowly than in the earlier experiment, since Mars has a mass nine times greater than that of the Moon. Also, we are now a good deal further from the sun (1.5 times to be exact), and Earth would immediately enter as an ice age worse than anything seen in geologic history. The oceans would completely freeze over, with ice eventually extending to great depth. Volcanic ashes and wind-blown dust would by and by cover the ice ocean and settle deeply into the ice. At the top of this mixture of rock and ice, there would be a thick, dusty layer, completely hiding the dirty ice below. Because of the low temperature, water content in the atmosphere would be very small. The composition of the atmosphere would change drastically, since it is being maintained by life activities. In the end, conditions on the Earth-turned-Mars might not look so different from those now prevailing on Mars.

Finally, let us move Earth into the orbit of Venus. Holding on to the atmosphere would be no problem, from the point of view of gravity, since Venus has 81.5% of the mass of Earth. However, it would become quite unpleasantly hot, since we are now much closer to the sun (about 1.4 times closer). The entire atmosphere would rapidly turn into a steam bath. Thick clouds would form in the upper atmosphere where the vapor would condense. The Earth would turn white and reject much of the sunlight. However, the steamy atmosphere would also act like a thick thermal blanket. The sunlight which reaches the surface would keep heating it, evaporating ever more water and making the atmosphere ever less transparent to outgoing heat radiation. This type of heating -- admitting sunlight but blocking back-radiation of heat till a certain temperature is reached -- is the well-known "greenhouse effect". In the orbit of Venus, the effect would be to keep heating the surface till the limestone rocks (which are made of calcium carbonate) would start disintegrating and give off carbon dioxide. Large amounts of this gas would now be added to the atmosphere. The result would be something like a wet Venus, perhaps not unlike a former state of that planet. Venus is in fact dry, presumably because the water it once had was split into hydrogen and oxygen, and the hydrogen has long since escaped into space.

These thought experiments, however fictional, illustrate that Earth's benign climate is rather delicately balanced between Ice Age and Hothouse conditions. One of the truly astonishing facts about the history of this Earth is that it has had an ocean for a very long time, at least 3,800 million years. Life needs water, and life existed more than 3.5 billion years ago. Hence, free water (that is, water not bound into rocks or minerals) must have been present uninterruptedly for most of the age of Earth (4.6 billion years). If the sun had been much colder or hotter in the past, the hydrosphere would either have frozen or boiled away into the atmosphere. We are lucky to have such a constant star, to be placed at the right distance to it, and to have the right size to hold on to our atmosphere and the ocean.